Lasers and amplifiers having tapered elements

ABSTRACT

A laser system for generating optical pulses at an operating wavelength of the laser system. The system has an optical resonator comprising first and second reflectors, and a tapered optical fiber disposed between the first and second reflectors. The tapered optical fiber has a core which has a tapered input section which tapers from single mode to multimode at the laser operating wavelength, an inner section of substantially constant diameter capable of supporting multiple modes at the laser operating wavelength. The tapered optical fiber can include a tapered output section wherein the core tapers from a first diameter to a second diameter that is smaller than the first diameter.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/644,424, entitled “Amplifying Optical DeviceHaving Tapered Gain Element”, filed 8 May 2012 and to U.S. ProvisionalPatent Application Ser. No. 61/793,534, entitled “Lasers and AmplifiersHaving Tapered Elements”, filed 15 Mar. 2013. The foregoing applicationsare incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to optical gain devices such as lasers andamplifiers, and more particularly, to optical fiber based amplifyingoptical devices, such as pulsed fiber amplifiers and pulsed fiberlasers, and to methods of making and using the same.

BACKGROUND

Fiber based lasers and amplifiers can act as “brightness converters”that convert high power, low brightness pump light to coherent, higherbrightness output light. Obtaining higher output powers—key for fiberbased devices to become even more competitive with conventional gas andsolid stated lasers—typically means delivering higher pump powers to thedoped core (typically rare earth doped) of the active fiber of the fiberlaser or amplifier. Higher power pump diodes or diode modules typicallyhave a multimode (MM) output that lacks sufficient beam quality toreadily directly couple to the small area core of a typical opticalfiber. Single mode (SM) pump diodes have better beam quality and candirectly couple, but typically have too low a power output.

A double clad (DC) fiber includes a larger inner cladding (“pumpcladding”) surrounding the smaller, active core and a second claddingabout the inner cladding. MM pump diodes can couple to the larger areaof the inner cladding/core and the pump light is absorbed by the activematerial in the core as the light propagates within the inner cladding.Pump light absorption (typically measured in dB/meter) is significantlyreduced compared to core pumping, and so the length of the gain fiber isincreased to provide for a total required absorption. For many devices,such as certain continuous wave (CW) devices, which may not involve ashigh power intensities as pulse devices, the increased length can be asmall price to pay for the increased output power, and the DC fiberrepresented a key advance.

Increased power output (e.g., one or more of average power, peak pulsepower, or pulse energy) of pulsed lasers and amplifiers is also of keeninterest. However, such devices are more complex, and obtainingincreased power output more problematic, especially as the pulseduration decreases, as the peak power can become quite high and theattendant high power intensity can trigger nonlinearities can limit theoutput power from an amplifying optical device, such as a fiber laser orfiber amplifier. Nonlinear effects can also limit the length of fiberwithin lasing cavities, making low repetition rate mode locked lasers(e.g., lower than 20 MHz) difficult to design. Pulsed devices, as wellas CW amplifying optical devices, could benefit from improvements.

SUMMARY OF THE INVENTION

Applicants have considered the problem of addressing nonlinearities soas to realize attendant benefits, such as providing higher powers and/orlower repetition rates in optical gain devices, with particular regardto pulsed optical fiber amplifiers, such as, for example, those based onrare earth active material (e.g., ytterbium (Yb)) and providing subnanosecond (1 ns or less) pulses.

Pulsed fiber lasers and amplifiers involve high peak powers that cantrigger nonlinearities that limit useful output power. Suchnonlinearities can include self-phase-modulation (SPM), stimulated RamanScattering (SRS) or stimulated Brillouin scattering (SBS). The strengthsof all of these nonlinear effects scale either linearly (for SPM) orexponentially (for SRS and SBS) in proportion to fiber length and pulsepower, and in inverse proportion to the mode area of the propagatingmode. There is often an interaction length associated with thesedeleterious nonlinear phenomena, meaning that either the threshold foronset is a function of the length of the gain fiber (and is higher forshorter lengths) and/or the amount of energy lost to nonlinear effectsremains below an unacceptable amount for the shorter lengths of gainfiber.

In considering the foregoing Applicants have discovered improvementsthat can provide for higher output power laser and amplifier opticaldevices, including, for example, methods and apparatus that can allowfor shorter lengths of gain fiber, as is discussed in more detail below.The shorter lengths can have higher thresholds for the onset of adeleterious nonlinearity, and hence allow higher output power. As willalso be evident below, other teachings are provided as well, including,for example, methods and apparatus directed to improved seed sources andnarrow bandwidth sources, including, for example, the use of longerlengths of fiber within lasing cavities, which allows lower repetitionrates. Generally, the teachings herein are considered of use inamplifying optical devices in addition to pulsed fiber devices, and, asregards fiber devices, in both core pumped and cladding pumpedapplications.

In one aspect of the invention, there is provided a laser system forgenerating optical pulses at an operating wavelength of the lasersystem. The system can include an optical resonator comprising first andsecond reflectors and a tapered optical fiber (TOF) disposed between thefirst and second reflectors. The tapered fiber can include a core havinga tapered input section which tapers from single mode to multimode atthe laser operating wavelength, a central or inner section ofsubstantially constant diameter capable of supporting multiple modes atthe laser operating wavelength and a tapered output section which tapersfrom a first diameter to a second diameter that is smaller than thefirst diameter.

Thus, in various embodiments, the tapered fiber has a core having asection of substantially constant diameter, which is arranged betweenthe tapered input section and the tapered output section. This sectionof substantially constant diameter may be referred to herein as an“inner section”, or a “central section”, or an “intermediate section”.

Various other features and aspects of the invention are now described.The features, aspects and practices described herein may be arranged inany combination with any of the other features, aspects or practicesdescribed herein, regardless of the particular exemplary embodiment inwhich such a feature, aspect or practice is described, except whereclearly mutually exclusive or a statement is explicitly made herein thatsuch a combination is unworkable. To avoid undue repetition and lengthof the disclosure, every possible combination is not explicitly recitedas a separate embodiment. The various embodiments of the inventionconsidered disclosed as within the scope of the invention are at leastas described in the multiply dependent claims appended hereto. As theskilled worker can ascertain, the methods of the present disclosure mayinclude any of the features, or steps relating to the function oroperation thereof, disclosed in conjunction with the description hereinof apparatus and systems.

The tapered output section of the core can taper from a diameter capableof supporting multiple modes to single mode at the laser operatingwavelength. The central section can comprise a large mode area fiber.

The taper of the tapered input and output sections can be such that thesingle mode guidance of the tapered sections is maintained during thetransition to the central section so that substantially only thefundamental mode propagates through the central section. The firstdiameter of the tapered output section in the system can besubstantially matched to the diameter of the central section. The gainmedium of the laser system can be comprised of the tapered opticalfiber. The gain medium can comprise a length of rare earth doped (RED)optical fiber separate from the TOF.

The laser system can further comprise a seed light source for inputtinglight into the optical resonator.

The TOF can comprise at least 80% of the length of the opticalresonator. The TOF can comprise at least 90% of the length of theoptical resonator. The length of the TOF in the laser system can bechosen to ensure that the repetition rate of the generated opticalpulses is no more than about 15 MHz, no more than about 10 MHz, no morethan about 5 MHz, or no more than 1 MHz. The optical resonator caninclude more than 40 meters (m) of optical fiber, more than 50 m ofoptical fiber, more that 75 m of optical fiber, or more than 90 m ofoptical fiber. The optical resonator can provide pulse have a pulseduration of 100 ps or less. The length of optical fiber comprised by theresonator can be approximately 100 m, the TOF can be approximately 98 mlong and the repetition rate of the generated optical pulses can beapproximately 1 MHz.

The first reflector can comprise a chirped fiber Bragg grating. Thesecond reflector can comprise a semiconductor saturable absorber mirror.One or more free space optical elements can be disposed between thetapered output section and a reflector, such as, for example, the secondreflector. The TOF can be composed of a single continuous piece offiber.

The second reflector can comprise a semiconductor saturable absorbermirror and one or more free space optical elements can be disposedbetween the tapered output section and the semiconductor saturableabsorber mirror. The TOF can be composed of a single continuous piece offiber.

The laser system can be arranged such that amplified spontaneousemission (ASE) power is no greater than 10% of the total optical poweroutput from said optical resonator. The system can be arranged such thatany optical power generated at the first Raman stoke shift is no greaterthan 10% of the total optical power output from said optical resonator.The generated optical pulses can have a spectrum with a fundamentalwavelength and the system can be arranged such that no more than 10% ofthe total optical power output from said optical resonator is outside ofa 30 nm bandwidth centered about the fundamental wavelength.

The laser system can be configured such that an optical signal intensitywithin the central section of the TOF remains high enough such that theamplified spontaneous emission (ASE) power is no greater than 10% of thetotal optical power output from said optical resonator and also whereinsaid optical signal intensity remains low enough such the optical powergenerated at the first Raman stoke shift is no greater than 10% of thetotal optical power output from said optical resonator. The system canbe arranged such that ASE optical power and Raman stoke shifted opticalpower corresponding to the first Stokes shift taken together do notaccount for more than 20% of the total optical power output from saidoptical resonator.

The tapered input section and/or the tapered output section may includea nonlinear taper profile. The tapered input section and/or the taperedoutput section may include a substantially linear taper profile. Thetapered input section and/or the tapered output section may include asubstantially exponential taper profile. In one practice of theinvention, the length of the tapered input section and the taperedoutput section combined can comprise no more than 10% of the totallength of the TOF.

The central section of the core of the TOF can have a refractive index(RI) profile which is substantially constant. The central section of thecore of the TOF has a refractive index (RI) profile taken relative tosilica wherein the percentage RI variation of the core RI according tothe formula [(maximum RI−minimum RI)/(2×minimum RI)]×100 is no greaterthan about 20%. In one practice, the variation according to theforegoing formula is no greater than about 15%. In another practice,variation according to the formula is no greater than about 11%.

In another aspect of the invention there is provided an optical fiberamplifier apparatus for providing short, high power optical pulsescomprising a TOF for amplifying optical input pulses having an inputsignal wavelength responsive to receiving pump light having a secondwavelength that is different than the input signal wavelength. The TOFmay comprise a RED core comprising a concentration of rare earthmaterial and may have an input end at which the core has a firstdiameter, an output end at which the core has a second diameter that islarger than the first diameter and a tapered length along which the corediameter increases from the first to the second diameter, where thetapered length is no greater than 250 cm and the core is single mode atthe input signal wavelength at the input end and multimode at the inputsignal wavelength at the second end. The optical fiber amplificationapparatus may be further configured such that the TOF includes anabsorption rate of pump light of at least 2.5 dB/meter and providesoptical output pulses from the output end having a time duration of lessthan 500 ns and a peak power of at least 100 kW.

The TOF may be adapted to provide the output pulses having an M² of nogreater than about 1.2. The optical fiber amplifier apparatus may bearranged such that amplified spontaneous emission (ASE) power is nogreater than 10% of the total optical power output from the output end.The optical fiber amplifier apparatus may be arranged such that anyoptical power generated at the first Raman stoke shift is no greaterthan 10% of the total optical power output from the output end. Theoutput optical pulses may have a spectrum with a fundamental wavelengthand wherein the optical fiber apparatus is arranged such hat no morethan 10% of the total optical power output from the output is outside ofa 30 nm bandwidth centered about the fundamental wavelength.

The tapered length may have taper profile arranged such that the outputsignal is substantially single mode and wherein the signal intensitytherealong remains high enough such that the ASE power is no greaterthan 10% of the total optical power output from the output end and alsowherein the optical signal intensity remains low enough such the opticalpower generated at the first Raman stoke shift is no greater than 10% ofthe total optical power output from the output end. The optical fiberamplification apparatus may be arranged such that amplified spontaneousemission (ASE) optical power and Raman stoke shifted optical powercorresponding to the first Stokes shift, taken together, do not accountfor more than 20% of the total optical power output from the output end.

The optical amplifier apparatus may be arranged such that the TOF issubstantially core pumped. The TOF may comprise a pump cladding and theoptical fiber amplifier apparatus may be arranged such that the TOF issubstantially cladding pumped. The optical fiber amplifier apparatus mayinclude an optical pump source. The optical pump source may be arrangedfor substantially core pumping the TOF and may have a substantiallysingle mode output.

The optical fiber amplifier apparatus may be configured such that theTOF includes an absorption rate of pump light of at least 5 dB/meter.The optical fiber amplifier apparatus may be configured such that theTOF includes an absorption rate of pump light of at least 9 dB/meter. Inone practice of the invention, the tapered length is no greater than 100cm. In another practice, the tapered length is no greater than 75 cm. Ina further practice of the invention, the tapered length is no greaterthan 50 cm. The tapered length may include a nonlinear taper profile,such as, for example, an exponential taper profile. The tapered lengthmay include a substantially linear taper profile. The tapered length mayextend along a longitudinal direction and the magnitude of the rate ofchange of the diameter of the core with respect to longitudinal lengthat a first location along the taper may be greater than the magnitude ofthe rate of change of the diameter of the core with respect tolongitudinal length at second location along the tapered length. Thediameter of the core may be larger at the first location than at thesecond location. The diameter of the core may be smaller at the firstlocation than at the second location. The diameter of the core at theoutput end may be least 1.5 times the diameter of the core at the inputend. The diameter of the core may increase substantially monotonicallyalong the taper length. The TOF may be drawn on a draw tower and thetapered length formed in a post-draw tapering process. The taperedlength may include a recoat section of cladding. The tapered length mayinclude at least a section that does not include a pump cladding.

The optical fiber amplifier apparatus may comprise a mode locked laserseed source in optical communication with the optical fiber amplifierfor providing the optical input pulses.

The optical fiber amplifier apparatus may be configured such that theoutput pulses have a repetition rate of at least 1 MHz. The opticalfiber apparatus may be configured such that the output pulses have arepetition rate of at least 10 MHz. The optical fiber amplifierapparatus may be configured such that the output pulses can have arepetition rate of at least 25 MHz. The optical fiber apparatus may beconfigured such that the output pulses can have an average power ofgreater than 1 W. The optical fiber apparatus may be configured suchthat the output pulses can have an average power of greater than 50 W.

The optical fiber amplifier apparatus may be configured such that theoutput pulses have a repetition rate of no more than 25 MHz. The opticalfiber apparatus may be configured such that the output pulses have arepetition rate of no more than 10 MHz. The optical fiber amplifierapparatus may be configured such that the output pulses can have arepetition rate of no more than 1 MHz. The optical fiber apparatus maybe configured such that the output pulses can have an average power ofgreater than 1 W. The optical fiber apparatus may be configured suchthat the output pulses can have an average power of greater than 50 W.

The optical fiber amplifier apparatus may be configured such that theoutput pulses have a picosecond time duration. The optical fiberamplifier apparatus may be configured such that the output pulsescomprise picoseconds pulse having a time duration of less than 100 ps.The optical fiber amplifier apparatus may be configured such that outputpulses have a time duration of less than 250 ps. The optical fiberamplifier apparatus can be configured such that the output pulses have apeak power of at least 500 kW. The optical fiber amplifier apparatus maybe configured can be configured such that the output pulses have a peakpower of at least 1 MW. The optical fiber amplifier apparatus can beconfigured such that the output pulses have a pulse energy of at least2.5 μJ. The optical fiber amplifier apparatus can be configured suchthat the output pulses have a pulse energy of at least 5 μJ. The opticalfiber amplifier apparatus can be configured such that the output pulseshave a pulse energy of at least 10 μJ.

In another aspect of the invention, there is provided a laser system forgenerating optical pulses at an operating wavelength of the lasersystem, the system having an optical resonator comprising: first andsecond reflectors; and a TOF disposed between the first and secondreflectors and having a core which has a tapered first section whichtapers from single mode to multimode at the laser operating wavelengthand a second section of substantially constant diameter capable ofsupporting multiple modes at the laser operating wavelength and havingan output end, wherein the length of the TOF is configured such that therepetition rate of the laser system is no more than 10 MHz.

The length of the TOF can configured such that the repetition rate ofthe laser system is no more than 5 MHz. The length of the TOF can beconfigured such that the repetition rate of the laser system is no morethan 1 MHz. The second reflector can comprise a semiconductor saturableabsorber mirror and the output end of the second section of the core ofthe TOF can be butt-coupled to the semiconductor saturable absorbermirror. The second reflector can comprise a semiconductor saturableabsorber mirror and the output end of the second section of the core ofthe TOF can be in optical communication with the semiconductor saturableabsorber mirror via one or more free space optical elements.

The output end of the second section of the core of the TOF can bespliced to, or continuously connected to, a tapered third section whichtapers from a first diameter to a second diameter that is smaller thanthe first diameter. The tapered third section of the core can taper froma diameter capable of supporting multiple modes to single mode at thelaser operating wavelength.

In yet an additional aspect, the invention provides an optical fiberthat can comprise a core and at least one cladding layer disposed aroundthe core. The optical fiber, at a first end, can have a first taperedsection in which the core tapers from a single mode diameter to amultimode diameter. The optical fiber can have a second tapered sectionwhich tapers from a first diameter to a second diameter that is smallerthan the first diameter at a second end. Between the first and secondtapered sections, the central core section can have substantiallyconstant diameter and be capable of supporting multiple modes. Therefractive index profile of the core of the optical fiber can besubstantially uniform.

The optical fiber can be configured to operate as a single mode fiberfor light having a wavelength of approximately 1064 nm. In certainpractices, the central core section of the optical fiber can have arefractive index (RI) profile taken relative to silica wherein thepercentage RI variation of the core RI according to the formula[(maximum RI−minimum RI)/(2×minimum RI)]×100 is no greater than about20%. The variation according to the foregoing formula in certainpractices is no greater than about 15%; in certain practices thevariation is no is no greater than about 11%.

In certain practices the central core section of the optical fiber has arefractive index (RI) profile taken relative to silica wherein thepercentage RI variation of the core relative to a RI of a shoulder ofthe core RI profile is according to the formula [(maximum variationRI−shoulder RI)/shoulder RI]×100 no greater than about 30%. In certainpractices the percentage RI variation is no greater than about 25%. Incertain practices the percentage variation is no greater than about 11%.

“Amplifying optical device”, as that term is used herein, means a laseror amplifier optical device. Picosecond optical pulses”, as that term isused herein, means pulses having a time duration of no less than 500 fs(femtosecond) and no greater than 1 ns (nanosecond); “nanosecond opticalpulses”, as that term is used herein, means pulses having a timeduration of no less than 500 ps (picoseconds) and no greater than 1 μs(microsecond). “Short optical pulses”, as that term as used herein,means pulses having a time duration of no greater than 500 nanoseconds;“ultrashort optical pulses”, as that term is used herein, means opticalpulses having a time duration of no greater than 500 ps.

Light, as that term is used herein, is not understood to be limited tovisible light but is used in the broader sense of opticalelectromagnetic energy.

Time durations, such as pulsewidths, and bandwidths as specified hereinare full width, half maximum (FWHM) time durations and bandwidths,unless otherwise noted.

A gain material, as that term is used herein, means a material that canprovide optical gain at a wavelength (referred to herein as a “gainwavelength”) responsive to being optically pumped at another wavelength(referred to herein as a “pump or pumping wavelength”). The gainwavelength may comprise the output wavelength of a laser cavity or anamplified input signal output by an optical amplifier. However, theconcept of optical pumping and gain is not limited to a laser oramplifier, and the term “amplifying optical device” (or “laser andamplifier optical device”) is used herein to include the broader classof devices that involve optical gain responsive to optical pumping.Typically, the optical gain is produced via a process of stimulatedemission responsive to a population inversion created by the opticalpumping. In each of the embodiments discloses herein, the optical gainmay be produced via a process of stimulated emission responsive tooptical pumping. The optical pumping may create a population inversion.

A gain material may comprise a RED material. RED material, as that termis used herein, means a material comprising one or more of the rareearths (typically as ions) such as, for example, one or more of theLanthanide elements of the periodic table (e.g., elements having atomicnumbers from 57 to 71). Erbium (Er), neodymium (Nd), holmium (Ho),thulium (Tm), and ytterbium (Yb) are all understood to be rare earthsthat are particularly useful in amplifying optical devices, such as, forexample, optical lasers, amplifiers, ASE sources or superfluorescentsources. A RED material can comprise more than one rare earth (e.g.,Er/Yb RED materials can be very useful). A gain material, however, neednot comprise a RED material. For example, a gain material may compriseTi-Sapphire, which is used in many solid state lasers.

The amplifying optical device may comprise an optical fiber, where theoptical fiber comprises the gain material. The gain optical fiber maycomprise a microstructured optical fiber. A pump source may comprise aselected RED material that comprises a rare earth comprised by the REDmaterial. The RED material may comprise one or more of holmium,neodymium, erbium, ytterbium or thulium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a fiber amplifier that can use atapered active optical element according to one embodiment of thepresent disclosure;

FIGS. 2A-2C schematically illustrates tapers for fibers of an opticalamplifier or laser, such as the fiber amplifier of FIG. 1, according tothe present disclosure;

FIG. 3 schematically illustrates examples of taper profiles for atapered active optical element according to the present disclosure;

FIG. 4 schematically illustrates an example of an “up-down” taperaccording to one practice of the present invention;

FIG. 5 shows another example of an up/down taper according toembodiments of the invention;

FIG. 6 shows another example of an up/down taper according toembodiments of the invention showing input and output sections;

FIG. 7A schematically illustrates a refractive index (RI) profile forconsideration as regards a tapered optical fiber (TOF) according to thepresent disclosure;

FIG. 7B schematically illustrates another refractive index (RI) profilefor consideration as regards a tapered optical fiber TOF according tothe present disclosure;

FIG. 7C schematically illustrates yet a further refractive index (RI)profile for consideration as regards a tapered optical fiber TOFaccording to the present disclosure;

FIG. 8 schematically illustrates a master oscillator and amplifierarrangement according to the present invention where the amplifierincludes a TOF including an up/down taper;

FIG. 9 illustrates an optical spectrum of an embodiment of the inventionin comparison to a prior art device;

FIG. 10 schematically illustrates an embodiment wherein an opticalresonator includes a TOF including an up/down taper;

FIG. 11 is plot of the reflectivity versus pulse energy for a saturableabsorber showing a nonlinear response;

FIG. 12 schematically illustrates an embodiment wherein an opticalresonator includes a TOF including an up/down taper where the output ofthe TOF is in optical communication with a saturable absorber;

FIG. 13 schematically illustrates another example of an opticalresonator including a TOF that includes an up/down taper; and

FIG. 14 schematically illustrates a further example of an optical devicehaving an optical resonator that includes a TOF.

FIG. 15 schematically illustrates an additional example of an opticaldevice having an optical resonator that includes a TOF including anup/down taper.

Not every component is labeled in every one of the foregoing FIGURES,nor is every component of each embodiment of the invention shown whereillustration is not considered necessary to allow those of ordinaryskill in the art to understand the invention. The FIGURES are schematicand not necessarily to scale. FIGS. 10, 12, 13, 14 and 15 illustratelaser systems having an optical resonator. In these figures, referencenumerals in which the last two digits are the same indicate the same orsimilar components, whose description is not always repeated herein.

When considered in conjunction with the foregoing FIGURES, furtherfeatures of the invention will become apparent from the followingdetailed description of non-limiting embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an optical amplifying apparatus 108that can comprise an input seed/pump section 168 and an opticalamplifier 158. The optical amplifier 158 comprises a tapered gainelement (TGE), which preferably comprises a length of rare earth doped(RED) tapered optical fiber (TOF). The optical amplifier 158 amplifiesthe input seed responsive to receiving the pump light, both of which maybe provided to the amplifier 158 by the input seed/pump section 168. Theinput seed/pump section 168 can comprise an input seed section 165 thatprovides the input seed signal, a pump section 167 that provides thepump light and a combiner 170 that combines the pump and input seedsignal for provision to the TGE amplifier 158.

The input seed section 165 can comprise any suitable input seed source.For example, the input seed section 165 and can comprise an oscillator112, which can comprise a mode locked fiber oscillator as describedelsewhere herein, and a downstream amplifier 114 and isolator.

The pump section 167 can comprise any suitable pump source 169. The pumpsection 167 can generate the pump light using a nonlinear process, suchas, for example, a FWM process.

Preferably the pump source section 167 comprises a substantially singlemode (SM) pump source, and amplifier 158 comprises a length of RED TOFthat is preferably substantially core pumped (as opposed tosubstantially cladding pumped) with substantially single mode pump lightfrom the pump source section 167. However, the RED TOF can be claddingpumped, and the TOF optical fiber can be few moded or MM along all or atleast part of its length.

The pump source section 167 of FIG. 1 is shown in a co-pumpingconfiguration with the TGE amplifier 158 and for illustrative purposesis shown as part of an input seed/pump section 168. However, the pumpsource section need not be part of an input seed/pump section. Forexample, the pump source section can be arranged to counter pump the TGEamplifier 158 from the output end of the TGE amplifier 158, in whichcase it would located to the right of the TGE amplifier 158 as shown inFIG. 1.

With reference to FIG. 2A (and as can equally apply to FIGS. 2B and 2C)a TOF is preferably formed from a rare-earth doped optical fiber havinga core that varies (e.g., increases) in diameter from the input end 271along its length to the output end 273. In the simplest form ofmanufacture, not only does the core diameter vary, but so too does theouter diameter of the fiber along the length of TOF. The length of theTOF can comprise some of the overall length of the RED fiber, at least amajority of its length, or substantially all of its length.

The length of TOF can have at its input end 271 a core 274 having adiameter D_(core-in). The core 274 can have the larger diameterD_(core-out) at the output end 273. The diameter of the core increasesalong at least some the length between the ends. The fiber can have acladding 276 (which can be the first cladding after the core, wherecladding refers to a region having an optical function of tending toconfine light to a region the cladding surrounds) that also tapers. Asshown in FIG. 2A, the cladding 276 can have a diameter at the input end271 of D_(clad-in) which can increase to a diameter at the output end273 of D_(clad-out). The input and output ends need not be transitionsto free space. For example, one or both of the input and output ends canbe locations selected along a length of fiber according to a criterionor criteria noted herein.

By way of example, a TOF according to the present disclosure can have ainput/output taper ratio (ratio of a diameter at the output end to thediameter of the same region (e.g., the core) at the input end of thefiber of at least 1.5, at least 2, at least 2.5, or at least 3. Invarious practices the taper ratio can be between (inclusive of endpointsof the stated ranges) about 1.5 and about 2, between about 2 and about3, or between about 3 and about 5. In various practices of theinvention, the foregoing recitations regarding taper ratios can apply toD_(core-out)/D_(core-in), or to the ratio D_(clad-out)/D_(clad-in), orto both of the foregoing ratios. In terms of actual diameters of fibers,a tapered fiber can have a core having a diameter that tapers from, forexample (again including endpoints), about 10 μm to about 20 μm, fromabout 10 μm to about 30 μm, from about 10 μm to about 40 μm, or fromabout 10 μm to > about 50 μm. The length of the TOF can be, in variouspractices of the disclosure, no greater than about 500 cm, no greaterthan about 250 cm, no greater than about 150 cm, no greater than about100 cm, no greater than about 75 cm, no greater than about 50 cm, nogreater than about 30 cm, or no greater than about 25 cm.

The length of the TOF, and particularly where TOF comprises and up/downtaper as described herein, can be, in various practices of thedisclosure, greater than about 5 meters, greater than about 10 meters,greater than about 20 meters, greater than about 50 meters, greater thanabout 75 meters or greater than about 100 meters.

Preferably the taper is configured such that the core is substantiallysingle moded (where “mode” refers to transverse modes, as the skilledworker is aware) at the input end 271. The core can be “large mode area”(LMA) fiber at its input end (and along the length of the taper). LMA,for the purpose of this disclosure, can mean a core numerical aperture(NA) of about 0.11 or less at 1060 nm and a core diameter of at least 10μm. In some practices, the core can have an NA of about 0.09 or less,again with a core diameter of a least 10 μm. In one practice, the fibertaper can have at it its input end a core diameter of 10 μm and can besingle mode at the input seed wavelength, and preferably LMA (10 μm,0.08 NA at 1060 nm) at the input seed wavelength, for example. However,this can equally be a smaller core or a larger core. The input end canalso support more than a single mode.

The taper of the TOF may be configured such that the input beam qualitydoes not substantially degrade along the length of the amplifier, eventhough the fiber tapers up in diameter such that the core can supporthigher order modes. In one practice, the length of TOF optical fiber isa single moded at its input end and the taper profile is such thatalthough the core increases in diameter along the taper such that thecore can support higher order modes, little or no optical energy istransferred into higher order modes. Accordingly, the length of TOF canprovide an output that is substantially in a single transverse mode andthat accordingly has good output beam quality. Beam quality can bemeasured and quantified according to the “M-squared” or “M2” parameter.As is explained in more detail below, the TOF can be single mode inoperation, although the TOF can have a structure, starting along somelength of the TOF, where the remainder of the length can in theorysupport the propagation of higher order modes.

The core diameter of a TOF optical fiber is preferably selected suchthat pulse-energy handling is not limited. However, equally, if anapplication does not require substantially single-transverse modeoperation, then taper and/or the output diameter can be such that thebeam quality is not maintained throughout the amplifier.

The profile of the taper can be substantially linear along the length.FIG. 2A schematically illustrates a linear taper. A taper can equallyhave a nonlinear or arbitrary taper profile. The taper can increasesubstantially exponentially along the length, as shown in FIG. 2B, orcan have a taper wherein the rate of taper is reduced along the lengthof the TOF, as is shown in the example of FIG. 2C. The taper profilealong the length of the fiber can be designed for improving, includingoptimizing, the performance of the amplifier in terms of nonlinearityand gain. For example, the effective nonlinear length of an amplifier isdefined by both the core-size and the gain profile along the fiberlength. By having a non-uniform taper profile (as, for example, is shownin FIG. 2B), the effective nonlinear length of the amplifier can be veryshort, since the highest gain of the amplifier occurs at a region of thefiber where the mode field is largest. The rate at which the mode fieldevolves along the length of the fiber can also affect beam properties ofthe amplified signal.

FIG. 3 schematically illustrates examples of taper profiles T1 and T2for a tapered active optical element (e.g., a RED TOF) according to thepresent disclosure. Increase in diameter is plotted as a ratio versusthe proportion along of longitudinal distance along the taper length interm of the total length L of the taper The tapers T1 and T2 shown inFIG. 9 are both nonlinear, can represent the diameter of the core of aTOF. For taper T1, the magnitude of the rate of change of the diameterof the core with respect to longitudinal length at a first location l₁nearer the input (and hence having a smaller diameter) is greater thanthe magnitude of the rate of change of the diameter of the core withrespect to longitudinal length at second location l₂ along the taperedlength, where the second location is nearer the output end and hence haslarger diameter. For taper T2 the opposite is true: the magnitude ofrate of change of diameter of the core with respect to longitudinallength along the taper is less at location l₁ than at location l₂.

An amplifying optical device according to a practice of the inventioncan include a tapered gain element for providing gain where gainelement, in a direction starting from the input end, includes an “up”taper, wherein a diameter of the gain element increases, followed by a“down” taper, where the diameter of the gain element then decreases. Forexample, such an up/down taper can include two of any of the TOF's shownin FIGS. 2A-2C (and can include taper profiles as shown in FIG. 3) wherethe two “output” ends as previously described are in opticalcommunication such the input end of the up/down TOF is the input end ofone of the tapers shown in FIG. 2 or 3 and the output end of thecombined up/down taper is what would have been previously described asthe input end of the of the other of the tapers, which is now arrangedas a “down” taper.

FIG. 4 schematically illustrates an example of an “up/down” taper 412according to one practice of the present invention. The up/down taper412 extends from the input end 417 to the output end 421. The up/downtaper can comprise an active (e.g. RED) TOF having a core 430 and acladding 436. As shown in FIG. 4, the up/down taper can include a middlesection 440, which may be substantially untapered, interposed betweenthe “up” taper 444 and the “down” taper 446.

Such an up/down TOF 412 may have two taper ratios—for example, a taperratio of the a diameter at the output end 421 (e.g., a diameter of thecore 430) to the diameter of the same region at the input end 417, aswell as a taper ratio of the maximum diameter of the region along thetaper to the minimum diameter of that region (e.g., the core 430). Themax/min ration will typically be larger than the output/input ratio. Forexample, in various practices of the invention, the max/min ratio may beat least 5 times, at least 4 times, at least 3 times, or at least 2times the output/input ratio. In various practices of the invention theoutput/input ratio is no greater than 3; no greater than 2.5, no greaterthan 2; no greater than 1.5; or no greater than about 1.25. In onepractice the output/input ratio is about 1. In various practices of theinvention any of the foregoing output/input ratios can be combined withany of the foregoing max/min ratios.

The overall length L of the TOF shown in FIG. 4 can be any of theoverall lengths noted above in the discussions of FIGS. 2A-2C. In somepractices the overall length can be twice that of those noted inconjunction with the description of FIGS. 2A-2C as the TOF of FIG. 9include both “up” and “down” tapers, (or even 2.5 or 3 times, to accountfor various lengths of L_(inner)). The lengths of L_(up) and L_(down)can be, but need not be, substantially the same. In various practices ofthe invention, the ratio of the length over which a diameter decreases(e.g., a diameter of the core region) to the length over which thediameter increases can be about 1, not greater than 0.075, not greaterthan 0.67, or not greater than 0.5. In other practices, the foregoingcan apply to the ratio of the length over which a diameter increases tothe length over which the diameter decreases.

In one embodiment, the TOF device is in operation able to guide only asingle mode of the waveguide, even though the waveguide properties ofthe inner region can in theory support higher order modes (V numbergreater than 2.405). The TOF device can be spliced at its input andoutput to single mode optical fibers with minimal optical splice loss,even though the fiber device has at least one region along its length inwhich the fiber waveguide has capability of supporting more than thefundamental mode, the transition from single mode to multimode is slowenough (adiabatic) so that the higher order modes are not excited.Preferably the device has an input tapered region from single mode fiberat the device input to multimode fiber and an output tapered region frommultimode fiber to single mode fiber at the output.

As is discussed in more detail below, a TOF device can be manufacturedon the draw during the fiber fabrication process. However, this devicecan be manufactured post-drawing of the fiber by using taperedmode-converters at both the input (Up converter) and output end (downconverter) of the fiber in which the region of multi-mode operationoccurs. Preferably the multi-mode length of fiber is continuous anduniform in waveguide properties. Preferably the length of the multi-moderegion of fiber (L_(inner)) is more than twice the length of the taperedsections of fibers, and preferably L_(inner) is more than 1 m in length.

FIG. 5 schematically illustrates a TOF device such as described aboveconjunction with FIG. 4, with the following features. The fiber device512 comprises an input end 517 having an input core section 542 andcladding section 543 providing waveguide properties that support only asingle mode (the fundamental mode) at a signal wavelength, and an outputend 521 with the output core section 544 and output cladding section 545providing waveguiding properties such that only a single mode issupported at the signal wavelength. Furthermore the central or innerregion 550 of the device has a core section 556 with a diameter largerthan the input core section 542 and output core section 544 and whereinthe central waveguide formed by core 556 and cladding 557 provides astructure that would support more than a single waveguide mode at asignal wavelength.

In one practice, the input end of a TOF device can have claddingdiameter of 100 μm, a core diameter of 10 μm and numerical aperture ofapproximately 0.08, supporting a single optical mode at 1064 nm signalwavelength. The outer diameter of the fiber can increases to 500 μm overa length of approximately 0.5 meters, resulting in a maximum corediameter of approximately 50 μm and a numerical aperture of 0.08. TheTOF device cladding diameter is maintained at approximately 500 μm for2.5 m along its length, after which it tapers down to an output endhaving cladding diameter of 100 μm and core diameter 10 μm, withnumerical aperture of 0.08, supporting a single waveguide mode at theoutput. The down-taper section of this device has a length ofapproximately 0.5 m.

The entire length of the TOF device can be fabricated from a singleoptical preform during the fiber drawing process and the entire deviceremains coated along its length, where the coating is applied to thefiber during the drawing process. It will be appreciated that therelative cladding diameters between input, output and central regions ofthe device can be different from those described within the specificexample above. It will also be appreciated that the device can bemanufactured from a single 500 μm diameter length of fiber, with inputand output tapers fabricated at the fiber ends during a process step orsteps after the 500 μm diameter fiber has been fabricated. It will alsobe appreciated that the input and output tapers can be fabricatedseparately and subsequently spliced to a uniform length of largediameter fiber to produce the device. It will also be appreciated thatthe relative and absolute lengths of the up-taper, down-taper andcentral sections of this device can be selected subject to therequirements of a given application.

In some examples, the TOF device of FIG. 5 has a total length L of atleast 20 m, or at least 50 m, or at least 75 m, or at least 100 m, ofwhich the central section of substantially uniform diameter constitutesat least 95%, or at least 97%, or at least 99% of the overall devicelength. In another example, the length L is approximately 4 m, with thecentral region having a core diameter of 100 μm and cladding diameter of1 mm over a length L_(inner) of 3.5 meters, with the up- and down-tapersections measuring only 0.25 m in length, having single mode 6 μm core(0.1 NA) and 10 μm (0.08 NA) waveguide at the input and output ends ofthe device respectively.

FIG. 6 illustrates another example of a TOF device as shown in FIG. 4 or5, illustrating elongated input and output single-mode fiber sections,respectively 551 and 555. The input single mode fiber section 551 can bethe output delivery fiber from an upstream element or component ormodule of an optical system and the output single mode fiber 555 can bethe input delivery fiber to a downstream element, component or module ofan optical system.

The input fiber 551 is spliced to or connected to the input single-modeend 541 of the device at a splice or connection 559A and the outputfiber 555 is spliced or connected to the output end 544 of the device atan output splice or connector 559B. In this example, the input fiberwaveguide structure (core 554 size and NA) is preferably substantiallysimilar to the input waveguide structure (core size and NA) of thedevice to minimize optical loss at the splice or connection. Similarlythe output fiber waveguide structure (core 558 diameter and NA) ispreferably substantially the same similar to the output waveguidestructure (core diameter and NA) of the device to minimize optical lossat the splice or connection.

The TOF devices described above can be doped with REDs. However, theycan also comprise substantially passive devices, where little or no gainis provided, such as, for example, because the dopants used to form thefiber waveguide device are largely devoid of, or do not include, rareearth elements and provide little or no optical gain to a transmittedsignal if or when pumped by pump light of a typical pump wavelength forRED fiber lasers and amplifiers. Substantially passive TOF devices canfind use, such as, for example, being incorporated into resonators, suchas cavity of a laser.

In other examples, the TOF device can be doped with one or more rareearth ions to make the device active and capable of providing opticalgain for a transmitted signal when pumped by laser light at a pumpwavelength. In this example, the device preferably has a double-cladwaveguide structure with the device, for example FIG. 5 having a coatingalong its length having refractive index lower than the inner claddingof the fiber. Preferably the device has a rare-earth dopant comprisingYtterbium to a sufficient concentration that the pump absorption at apump wavelength exceeds 2 dB per meter.

It is preferred that a tapered optical element, such as TOF, have asubstantially uniform refractive index profile in the central region, soas to avoid concentration of optical energy and possible attendantgeneration of nonlinear affects (e.g., more power at the first Ramanstoke shift) or more power into higher order modes. FIGS. 7A-7C showpossible refractive index (RI) profiles for fibers considered forfabrication of RED TOFs for use for amplifying optical signals. The RIprofile plots the difference between the RI at a location and the RI ofpure silica (typically the cladding comprises pure silica), not theabsolute value of the RI of the core region (which is typically dopedsilica, and hence can be in the region of 1.45 at a wavelength of 1micron).

FIG. 7A shows an example of a preferred RI profile 712, while FIGS. 7Band 7C show less preferred core RI profiles. The section of the RIprofile corresponding to the core can be identified as the regionbetween the shoulders 720A and 720B in FIG. 7A, between the shoulders730A and 730B in FIG. 7B, and between the shoulders 740A and 740B inFIG. 7C. With reference to FIG. 7A, the sections of the RI profile 712corresponding to the cladding disposed about the core are identified byreference numerals 750A and 750B; similar sections are shown in FIGS. 7Band 7C though not specifically identified by reference numerals.

Though it is preferable that the RI profile of the core be as uniform aspossible, it is understood that in any practical fiber there will oftenbe variations. With reference to FIG. 7A, one measure of the variationis the percentage variation according to the formula for the maximum andminimum variation within the core section of (max RI−min RI)/(2×minRI)×100. By this measure, the core of FIG. 5A has a percentage variationof [(0.0023−0.0018)/(2×0.0018)]×100 of about 11%. Using the sameformulation, the core RI profiles of FIGS. 7B and 11C have variation of,respectively, about 35% and about 21%.

In various practices of the invention, the variation of the RI withinthe core, as determined in percentage terms by the foregoing formula, ispreferably is no greater than about 20%. More preferably the variationis no greater than about 15%, and most preferably the variation is nogreater than about 11%.

Percentage variation in RI profile does not fully demonstrate thedifferences between the RI profiles of FIGS. 7A, 7B and 7C. Note thatwhereas FIG. 7A show a core RI profile having a ripple in between theshoulders 720A and 720B, the RI profile 755 of FIG. 7B shows a coresection having a generally depressed section 760 relative to theshoulders 730A and 730B and the RI profile 770 of FIG. 7C shows a coresection having a generally raised inner section 775 between the betweenthe shoulders 740A and 740B bounding the core. The depressed innerregion 760 can result from dopant burn-off during collapse of an MCVDpreform from a tube to a solid rod. This burn-off can be compensated byflowing appropriate dopants through the center of the tube, however overcompensation can generate the raised region 775. A measure, inpercentage terms, of the maximum magnitude of the variation of the RIprofile from a shoulder of the core relative to the shoulder RI canbetter quantify the presence of the depressed or raised region in the RIprofile of the core. It is most desirable that variation in RI asdetermined by the foregoing criterion, is no greater than about 30%, ormore preferably no greater than about 25%, or more preferably no greaterthan about 15%. For the RI profiles shown in FIGS. 7A-7C, the variationis according to this criteria estimated to be about 11% for FIG. 7A andabout 41% for FIGS. 7B and 7C. As an example of the calculation, theshoulders of FIG. 7B have an RI of about 0.0034. The maximum magnitudeof the variation is absolute value [0.0020−0.0034]=0.0014 and thepercentage variation relative to the shoulder is [(maximummagnitude)/0.0034]×100=about 41%.

RI profiles are typically taken at a wavelength of about 633 nm.

Tapering fibers is known, particularly in the manufacture of couplers.Tapered double clad (DC) fibers are also known. A tapered DC fiber canbe tapered as it is drawn from a draw tower. The speed of the fiber drawprocess is varied during the draw process, such that the diameter of theresulting fiber changes along the length of fiber. See for example PCTWO2009043964A. See also Double Clad Tapered Fiber for High PowerApplications, V. Filippov et al., Optics Express, Vol. 16, No. 3, 4 Feb.2008, pp. 1929-1944.

As previously described, DC fibers are not always ideal and cantypically only be used in relatively long lengths (>1 m) due toabsorption limitations—the absorption of the amplifier is dependent onthe cross-sectional ratio of the RED core and inner cladding. Inaddition, tapering fibers during the draw process is difficult,requiring modification of the draw speed very quickly. The speed of thisprocess change generally limits the length of the taper to a minimum ofabout a couple of meters.

Using a core-pumped approach, the absorption is high and the fiberlength can be very short (as already mentioned). Furthermore,core-pumping, unlike cladding pumping, need not require anoptical-quality outer surface of the fiber. This is important fortapering since it allows the use of post-fiber-fabrication taperingprocedures, wherein the fiber is drawn and coated, and subsequently thefiber coating is removed and the fiber tapered, before re-coating.

The post draw process may include heating the fiber in a flame or plasmaor heated crucible, while applying a tensile force to stretch the fiberin a controlled way. Long (> a few mm) tapers can be produced bytraversing a flame along the fiber while applying tension. Applying anon-uniform tension over time allows control of the taper profile alongthe fiber length. Using this approach, tapers can be controlled and madeover lengths from a few mm to greater than 1 m, therefore it is an idealapproach to producing a TOF over very short (<0.5 m) lengths. As anexample, an Yb-doped fiber (preferably fabricated using aphosphor-silicate or alumina-phosphor silicate host material in the corefor enhanced robustness to photodarkening, is fabricated in aconventional way. The fiber has a core diameter of 40 μm, claddingdiameter of 400 μm, and a numerical aperture of approximately 0.08. Intheory, the fiber can support several modes within the operativebandwidth of Yb.

After fabrication, the protective coating and/or cladding (typically apolymer) may be stripped from a length of the fiber. Some fibers used acombined protective coating/cladding, and often in such a case theprotective coating has a lowered index of refraction such that it canserve as a pump cladding. Stripping this protective coating can also beconsidered as stripping the second cladding. Other fibers may haveseparate protective coatings and claddings, such as, for example, apolymer protective coating over a glass first cladding. In this case theprotective coating is stripped and the cladding left intact. An allglass fiber may not require any stripping of a coating/cladding at all.

After any stripping operation, the fiber is tapered by applying atensile force while traversing the flame along the fiber length. Thetension applied as a function of time, is chosen to produce auniformly-tapered fiber with a profile as shown in FIG. 2A. For example,the fiber can be tapered over a length of 30 cm to a diameter of 100 μm.The fiber can recoated with cladding or a protective coating after thepost draw tapering process. The fiber can fabricated into an amplifier,having an input fiber with 10 μm core, 100 μm diameter and core NA of0.08, supporting only a single transverse electric mode. The outerdiameter can increase along the 30 cm length to 400 μm at the amplifieroutput, wherein the core diameter is 40 μm, with NA of 0.08.

Such TOF device, such as RED TOF amplifier, can have very lownonlinearity and, since the change in fiber core size is gradual, thesingle mode of operation is maintained along the length of theamplifier. The fiber can be re-coated and can be packaged to be linearor coiled, depending on the required application.

Thus in one embodiment, there is provided an up/down TOF opticalamplifier comprising a single mode RED optical fiber pumped by one ormore pump laser diodes said RED optical fiber having an input end and anoutput end, both of which can be spliced to single mode optical fiberswith minimal optical splice loss and wherein the TOF device has at leastone region along its length in which the fiber waveguide has capabilityof supporting more than the fundamental mode. However, the amplifierremains single mode, as in operation higher order modes are not exciteddue to one or both of the up/down tapers to single mode. This singlemode optical amplifier is preferably configured within a masteroscillator power amplifier (MOPA) configuration, where the MOPApreferably delivers short or ultrashort pulses and wherein the up/downTOF amplifier provides single-mode amplification with reduced opticalnonlinearity over single mode amplifiers with uniform waveguideparameters along its length. As the skilled worker can ascertain fromthe disclosure herein, the such single mode amplifier can be single cladas well double clad, and core pumped as well as cladding pumped.

FIG. 8 schematically illustrates an example of a modelocked MOPAincorporating a TOF device, such as, for example, and up/down TOF devicesuch as illustrated in FIGS. 4-6. The MOPA can deliver ultrashortoptical pulses, typically below 100 picoseconds in duration. However,such a system can also be used to generate continuous wave light andnanosecond (>100 ps) optical pulses. The optical fiber amplifier 832comprises a RED, single-mode up/down TOF device 834, such as shown abovein FIG. 6, having single-mode input waveguide and single mode outputwaveguide, with a very large mode area central section having waveguideparameters (core size, numerical aperture, etc.) which would ordinarily(based on V-number alone) allow for the excitation of higher orderwaveguide modes.

The MOPA 860 comprises a passively modelocked optical fiber oscillator831 which delivers pulsed light at a wavelength of 1064 nm with pulsesof between 1 picosecond in duration and 100 picoseconds in duration, ata pulse repetition rate in the region of tens of MHz and average powersof a few mW (pulse energy in the range of tens of pJ). The output of theoscillator passes into an Ytterbium doped, double-clad optical fiberpre-amplifier 833 which is pumped by a multi-mode laser module 835. Thepre-amplifier 833 amplifies the oscillator output to approximately 100mW average power and a corresponding pulse energy of the order of 5-10nJ, with peak power in the region of 1 KiloWatt.

After passing through an optical isolator 837, the pre-amplifier outputis injected into a double clad Ytterbium-doped fiber power amplifier 832comprising an up/down TOF pumped by another multimode pump laser module835. The output fiber of the isolator 837 is a single mode fiber withcore diameter approximately 10 μm and numerical aperture approximately0.08. The up/down TOF double clad amplifier in this example can comprisea up/down TOF as showing in FIGS. 4-6. By way of example, the poweramplifier comprises a fiber of geometry similar to FIG. 5 of thispresent invention although it will be appreciated that a device similarto FIG. 4 or 6 can also be utilized.

The total length L of the device is 3.3 m long comprising; a 2.3 m longcentral uniform section (550 in FIG. 5) having outer diameter of 250 μmand core diameter of 20 μm (resulting in a mode field diameter ofapproximately 18 m) and input and output tapered sections, each of 0.5meters in length, resulting in input and output cladding diameters (543and 545 respectively) each of 125 μm and input and output core diameters(542 and 544 respectively) each of 10 μm. The numerical aperture of thewaveguide throughout the fiber is approximately 0.08 resulting in asingle mode input and single mode output fiber section of the amplifierdevice.

The amplifier TOF device in this example is fabricated during the fiberdrawing process as a single homogenous piece of fiber, ensuring that thetransition from the single mode input to the very large mode areacentral section and the transition from the central section to thesingle mode output fiber is smooth and therefore does not promote thecoupling of energy from the fundamental mode of the single mode inputfiber to higher order modes of the central region of the fiber and as aresult, maintains a low-loss transition at the output end of the device.It will be appreciated that this amplifier device can also be fabricatedpost-fiber draw, by splicing tapered sections of fiber at both the inputand output end of the central region of the amplifier.

The output of the isolator 837 is connected to the input of apump-signal combiner (not shown) which can be a single-clad wavelengthdivision multiplexer (WDM) or a double-clad single-mode plus multimodepump combiner such as a tapered fiber bundle or side-coupler commonlyavailable from companies including Gooch and Housego and ITF forexample).

In this specific example, a double-clad tapered fiber bundle is used tocombine the signal from the isolator 837 and the pump light from themultimode pump laser module 835. The output of the pump combiner is adouble clad fiber having core and cladding parameters (diameter and NA)matched to those of the input of the up/down TOF amplifier device 832already described. The output of the pump combiner is spliced at 832A tothe input of the amplifier device with low loss for both the signal andpump light. The dopant concentration of Yb in the core of the amplifierdevice is such that the double clad fiber has a pump absorption of 2.5dB per meter when pumped with multi-mode laser diodes at 915 nmwavelength. The total pump absorption of this amplifier is in the regionof 8 dB.

The output of the double clad amplifier fiber is spliced at 832B to apassive, single-clad optical fiber with 5 μm radius core and NA of 0.08,making it well matched to the Yb-doped fiber and therefore having verylow loss at the splice 832B.

This particular MOPA is suited to generation of optical supercontinuumwithin photonic crystal fiber (PCF), also known as holey fiber ormicrostructured optical fiber. In the example of FIG. 8, a length of PCF836 with zero dispersion wavelength approximately 1 μm and anomalousdispersion at the amplifier signal wavelength of 1064 nm, generatesspectral broadening and supercontinuum generation due to nonlinearinteraction between the amplifier pulsed laser source from the amplifier832. In this example, the PCF 836 is spliced at 838 to the passive fiber839. The single mode nature of the passive fiber makes possible a lowsplice loss to the PCF.

FIG. 9 shows the two spectra—that of a prior art amplifier (dashedcurve) and an amplifier as per this invention (solid line)—where bothamplify ultrafast pulses to nominally the same peak power. The dashedcurve is the optical spectrum of the pulses from a prior art MOPA at theoutput of the amplifier (i.e., amplifier 832, but with a uniform fiberinstead of up/down TOF) at an average power of approximately 3.4 Watts,a pulse repetition rate of 20 MHz (pulse energy 170 nanoJoules) and apeak power in the region of 35 kiloWatts. More particularly, in the caseof the prior art MOPA, the design is as shown in FIG. 8 except that thepower amplifier 832 comprises a 3.3 m long single-mode, Yb-doped doubleclad fiber, with uniform outside diameter along its length. In thisspecific example, the Yb-doped fiber has an outer diameter of 125 μm anda core radius of 5 μm with a numerical aperture of approximately 0.08rendering it single mode at 1064 nm. The dopant concentration of Yb inthe core is such that the double clad fiber has a pump absorption of 2.5dB per meter when pumped with multi-mode laser diodes at 915 nmwavelength. The total pump absorption of this amplifier is in the regionof 8 dB. The output of the double clad amplifier fiber is spliced 832Bto a passive, single-clad optical fiber 839 with 5 μm radius core and NAof 0.08, making it well matched to the Yb-doped fiber and thereforehaving very low loss at this splice.

The solid curve shows the an example of the optical spectrum of thepulses from the MOPA at the output of the amplifier 832 including theup/down TOF also at an average power of approximately 3.4 Watts, a pulserepetition rate of 20 MHz (pulse energy 170 nanoJoules) and a peak powerin the region of 35 kiloWatts.

The prior art dashed line spectrum shows distinct features. As can beseen at (i), the input signal pulse is centered at 1064 nm with narrowbandwidth and this has broadened during amplification due toself-phase-modulation (SPM). As can be seen at (ii), the peak power inthis amplifier is sufficiently high to significantly exceed thethreshold for stimulated Raman scattering (SRS) and the spectrum showsthe first Stokes component at a wavelength of approximately 1020 nm.

In generating supercontinuum or in delivering pulses for materialsprocessing, the presence of the Stokes spectral components isundesirable since this component of the pulse (in the dashed lineexample this is a reasonable percentage of the total pulse energy) doesnot contribute significantly to the supercontinuum generation processand can have deleterious effects on any materials processingapplications. The peak power that can be delivered with the prior artamplifier has significant disadvantages.

In summary, clearly, in the case of the uniform fiber, prior artamplifier the nonlinear effects are significantly larger than those ofthe up/down TOF amplifier. This is shown by the presence of spectralcomponent at 1120 nm (ii) due to stimulated Raman scattering as well asthe significantly broader signal pulse (i) compared to that of the solidcurve of the tapered amplifier as per this invention (iii) due to alarger amount of nonlinearity (self phase modulation)

It will be appreciated that the level of nonlinearity in the amplifierof this present invention can be further reduced by increasing the coremode-field diameter of the central region or inner region of the TOFamplifier device of FIGS. 4-6 utilized within the MOPA of FIG. 8. Forexample, by having a 500 μm cladding, 40 μm core, the mode fielddiameter in the fundamental mode can be substantially increased, vastlyreducing the impact of nonlinear effects during amplification within theamplifier.

The reduced nonlinearity provided by the invention can provide majorbenefits in enabling delivery of clean optical pulses with higheroptical peak power. This has advantages in many specific applications,including by way of example, supercontinuum generation, in particular byminimizing nonlinear effects such as SRS generation, the supercontinuumgeneration process becomes increasingly more efficient. There are alsobenefits in Four Wave Mixing (4WM) generation in photonic crystal ormicrostructured optical fibers—4WM requires a narrow spectral bandwidthof the pump laser in order to efficiently exploit parametric processeswithin the nonlinear fiber. By minimizing spectral broadening due toself-phase modulation, the amplifier device of this invention allows forthe delivery of higher peak powers for a given spectral linewidth, thusincreasing 4WM system efficiencies. Finally, there can be advantages fornonlinear frequency conversion including second harmonic, thirdharmonic, etc. Harmonic generation in nonlinear crystals typicallyrequire a narrow spectral linewidth in order to maximize conversionefficiency. The novel amplifier device of this invention allows forvastly reduced nonlinear effects, enabling higher peak powers to bedelivered to the nonlinear crystal for a given spectral bandwidth of thepulses.

In another embodiment of the invention, a TOF device, such as an up/downTOF device, is utilized within an optical fiber laser cavity in orderminimize nonlinear optical effects within the laser cavity.

FIG. 10 is a schematic representation of a passively modelocked fiberoscillator device. The passively modelocked fiber oscillator 1000comprises a single mode pump laser diode 1005 operating at approximately976 nm, delivering up to 300 mW average power and coupling light intothe cavity via a fiber wavelength division multiplexer (WDM) component1007. The cavity or resonator comprises two end reflectors—a chirpedfiber Bragg grating (CFBG) 1013 and a semiconductor saturable absorbermirror (SESAM) 1015, where the SESAM has the dual role of forming acavity end reflector and an intensity discriminator which enables andmaintains the formation of the short pulse within the cavity. The cavityfurther comprises a length of single-clad Yb-doped optical fiber 1022which provides gain within the laser cavity, and a low nonlinearityup/down TOF device 1050 which makes the cavity the desired length toproduce a fixed pulse repetition rate for the cavity, which is inverselyproportional to the distance between the CFBG and SESAM end reflectors(the cavity length).

Some discussion of operation of the cavity is of use. The cavityproduces ultrashort optical pulses at a wavelength determined by thereflectance spectrum of the CFBG 1013, which fits within the spectralgain bandwidth of the Yb-doped fiber gain material 1022. The pulse widthof the cavity is determined by a number of parameters including thedispersion of the CFBG and the response time of the SESAM.

Pulse duration τ in mode-locked fiber lasers is largely dictated byintracavity dispersion D_(net) and in first approximationτ˜D _(net) ^(1/2).

For reliable self-starting the laser should operate in the regime ofanomalous dispersion and therefore for operation in the 1064 nm spectralrange one has to use dispersion compensation to bring net intra-cavityto a positive value (anomalous dispersion). In many cases a chirpedfiber Bragg gratings (CFBG) acts as a dispersion compensator andtherefore net dispersion can be written asD _(net) =Z _(c) D _(f) +D _(CFBG),where z_(c) is the cavity length in meters, D_(f)=−40 fs/(m nm) is fiberdispersion (in the 1064 nm spectral region). When net dispersion of alaser cavity is anomalous then the laser produce opticalsolitons—nonlinear pulses with strong relation between peak power andpulse duration which can be written asI _(p)τ²=Const.Where I_(p)=P_(p)/A_(eff) is pulse intensity, P_(p) is peak power andA_(eff) is effective core area. From last relation it becomes clear thatnet intracavity dispersion controls not only pulse width but also peakpower and the longer the cavity the lower peak power required tomaintain stable mode-locking (because dispersion of CFBG is usuallygreater than fiber dispersion). What is also clear is that the greaterthe effective mode area the greater the peak power of the generatedpulses and thus for given pulse intensity LMA fibers offer certainadvantages since it allows to achieve stable mode-locking at lowerrepetition rates.

When a mode-locking mechanism is based on SESAM technology, theintensity of the generated pulses should be high enough to saturate(bleach) the semiconductor absorber as it is illustrated in FIG. 11below where E_(sat)=P_(sat)τ, and for reliable mode-locking P_(p) shouldbe greater than P_(sat) but not too much otherwise it could result inunwanted effects.

Furthermore, the peak pulse intensity may also be limited by the onsetof non-linear effects such as self-phase modulation. Self-phasemodulation Φ_(nl), may be calculated according to:

$\Phi_{nl} = {\frac{2\pi\; n_{2}{PL}}{\lambda\; A} \leq \pi}$

Self-phase modulation nonlinear phase shift, Φ_(nl) is a function of thenon-linear refractive index n₂, which in silica optical fibers isn₂=3.2*10⁻²⁰ m²/W; peak power P; fiber length L; operational wavelengthλ; and the area A of the fundamental mode in the fiber. Self-phasemodulation should be less than or equal to π in order to preventsignificant distortion of the frequency spectrum of the output pulse.Applicants have found that it can be desirable to limit the nonlinearphase shift in a fibre mode locked laser resonator to less than or equalto π, and in particular where the mode locked laser operates with thegain bandwidth of ytterbium, and more particularly where fiber comprisedby (e.g., within) the resonator, such at the fiber of an TOF device,such as an up/down TOF device that includes a substantially untaperedinner region, has normal (negative) dispersion at the operatingwavelength of the mode locked laser.

Typically, in the embodiments of the inventions described herein, theoptical fiber of a TOF device, whether just up taper, down taper orup/down taper, and/or the majority of the length of fiber comprised by aresonator or amplifier, will provide normal (negative D, in ps/nm-km)dispersion at the operating wavelength of the device. This can result insignificant differences in terms of design and/or operation as comparedto devices some or most of the length of fiber provides (positive D)dispersion.

All the above indicates that the use of large mode area fibers arebeneficial for stable mode-locking. Also it is clear that independentcontrol of intensities at SESAM surface and inside cavity fiber is alsobeneficial for stable operation. It is therefore preferable within thistype of cavity as well as within other applications, to have a singlemode optical fiber cable with low nonlinearity and a high stabilityagainst bend losses and coupling to any higher order modes supportedwithin the waveguide. The cavity example of FIG. 10 is one such case.

In the example of FIG. 10, the up/down TOF device 1050 providessubstantially less optical gain than the active optical fiber 1022, suchas less than 40%, or less than 25%, or less than 10% of the optical gainprovided within the resonator or cavity. Preferably the up/down TOFdevice 1050 is passive (substantially free of rare-earth dopants andhence substantially gain free) but as noted above an active device couldalso be used in some configurations. The entire resonator is preferablyfabricated from polarization maintaining fibers butnon-polarization-maintaining fibers can also be used. Furthermore, incertain configurations, additional optical components including, forexample, polarizers, faraday rotators, waveplates, attenuators,modulators etc can also be included within the cavity.

In one example embodiment of the present invention, the length of fiberin the cavity length is 10 meters, resulting in a pulse repetition rateof approximately 10 MHz. The length of the low nonlinearity opticalfiber device is approximately 8 meters, comprising 80% of the cavity.The device has a single mode fiber input and output, said fiber havingcore diameter of approximately 10 μm and core NA of 0.08. The device hascentral region forming the majority of the device length (7 m) andhaving core diameter of approximately 20 μm and waveguide NA of 0.08.

In another example embodiment, the cavity includes a total length offiber of 100 meters, resulting in a pulse repetition rate ofapproximately 1 MHz. The length of the low nonlinearity optical fiberdevice is approximately 98 meters, comprising 98% of the cavity. Theup/down TOF device 1050 has a single mode fiber input and output, saidfiber having core diameter of approximately 10 μm and core NA of 0.08.The up/down TOF device has central region forming the majority of thedevice length (96 m) and having core diameter of approximately 50 m andwaveguide NA of 0.08.

In prior art laser cavities, comprising non tapered single mode opticalfibers, nonlinearity within the waveguide restricts the length of thefiber cavity and hence limits the minimum pulse repetition rate that canbe delivered by a passively modelocked laser cavity to 10's of MHz.Further reduction in repetition rate requires the use of pulse pickercomponents after the cavity as well as additional gain to compensate forloss in the pulse picker components. These additional components add tothe material cost, system complexity and system size. It is therefore anobjective to produce a low pulse repetition rate modelocked oscillatorhaving compact form factor and minimum complexity. This is enabled bythe use of ultra-low nonlinearity single mode optical fiber devicesaccording to specific embodiments of the present invention.

It will be appreciated that, within FIG. 10 and within exampleembodiments described above, the oscillator is configured with a SESAMas a saturable absorber. In all of these embodiments, it will beappreciated that a different type of saturable absorber can be utilized,for example based on carbon nanotubes, graphene, nonlinear polarizationrotation etc.

Similarly it will be appreciated that, whilst the specific cavityarchitecture described in FIG. 10 and within previously describedembodiments includes having the saturable absorber mirror butt-coupleddirectly to the cavity fiber, one can also configure the optical powerof the cavity onto the saturable absorber using free space optics. Forexample, by collimating and then focusing the light onto the saturableabsorber mirror, one can select the appropriate optics in order togenerate the desired optical spot size and necessary intensity on thesaturable absorber required to initiate and attain stable modelocking.

FIG. 12 shows an example embodiment cavity 1200 wherein the output fromthe up/down TOF device 1250 is launched onto the SESAM 1215 by lenses,in this example comprising two lenses 1291 and 1292, which collimate andfocus the light respectively. Reflected light from the SESAM 1215 islaunched back into the up/down TOF device 1250 with low optical loss ifthe lenses are suitably selected and aligned.

In another example embodiment, shown by way of example in FIG. 13, anoptical cavity 1300 comprises an up/down TOF device 1350 which is dopedwith an RED material such as Ytterbium, Erbium, Thulium, neodymium or acombination thereof. The up/down TOF device provides reduced opticalnonlinearity within the cavity as described in previous embodiments, butalso providing a substantial amount of the gain within the cavity. Inthis example, the TOF device can have the geometrical properties ofpassive or low gain TOF devices previously described in otherembodiments of this current invention. The active up/down TOF device1300 has an input section which is single mode at the laser operatingwavelength, a central optical fiber section which has large mode areaand could potentially support a plurality of optical modes, and anoutput optical fiber section which is single mode at the wavelength ofoperation of the cavity. Preferably the TOF device is one continuouspiece of fiber but equally this can comprise more than one section offiber spliced together to achieve the described physical and opticalproperties of the TOF device.

The active up/down TOF device 1350 is doped with a concentration of rareEarth ions suitable to absorb sufficient pump laser diode 1305 photonssuch that the device provides more gain at the optical cavityoperational wavelength than the combined loss of the cavity, includingthe loss of the up/down TOF device. For example, if the TOF device isvery long (for example greater than 50 m, such as, for example, about100 m) in order to produce a very low pulse repetition rate, if therare-Earth dopant level is too high, then all of the pump photons fromthe cavity pump laser diode 1305 will be absorbed in a relatively shortlength of the TOF device, resulting in a substantially long length ofun-pumped rare-Earth doped optical fiber which will add significantunwanted loss to the cavity. Like end reference numerals in FIG. 13 tothose of other FIGURES showing cavities indicate similar components,whose description is not always repeated. For example, reference 1313 inFIG. 13 indicates the same or similar component as reference 1013 inFIG. 10, i.e., a chirped fiber Bragg grating. The same considerationapplies to the other FIGURES showing cavities.

FIG. 14 shows another example embodiment of a modelocked optical fibercavity according to the current invention. The cavity 1400 is similar tothe cavity of FIG. 10 with the following exception. The up/down TOFdevice 1450 of this cavity, shown as a substantially passive fiberproviding very little if any optical gain, has an input section which isa single mode fiber at the operational wavelength of the cavity and alength of large mode area optical fiber which has potential to guide aplurality of optical modes. As with all previously described embodimentsof TOF devices, the region between the single mode and potentiallymultimode sections of the device is tapered in such a way that thesingle mode guidance in the single mode fiber section is maintainedduring the transition to the large mode area section of the device,resulting in propagation of substantially only the fundamental modethroughout the of the fiber device. The output 1493 of the up/down TOFdevice 1450 in this embodiment of the current invention is not tapereddown or is tapered down to a diameter greater than that of the inputsingle mode section of the device, such that the output optical fiber islarge mode area and can potentially support a plurality of opticalmodes.

In this example, the output optical power from the TOF device islaunched onto a SESAM 1415 via an arrangement of optical lenses 1491,1492 with which the desired spot size for achieving and maintainingmodelocking can be established through selection of the individuallenses. It will be appreciated that the SESAM could also be butt coupledto the output of the TOF device.

In FIG. 14, the output of the TOF device is shown as a cleaved fiber.This end face of this device preferably provides minimal back reflectionof light back into the device and can be achieved with standard methodsincluding angle-cleaving or suitable use of anti-reflection coatings orend capping of the device.

In all these mentioned embodiments of modelocked optical cavities, thearchitecture is shown in a similar way, having pump light from a pumpdiode enter the cavity through a Fiber Bragg grating which can bechirped or uniform. These cavities are also shown with the saturableabsorber element positioned at the output of the TOF device.

It will be appreciated that, within the spirit of this invention, otherconfigurations of modelocked fiber cavities can be built based ontechniques known in the art, but the fundamental use of a tapered, largemode area optical fiber device which supports substantially only thefundamental mode to reduce optical nonlinearity and provide means toincreasing cavity length, reducing pulse repetition rate is achieved.

By way of example, FIG. 15 shows one such example architecture of amodelocked optical fiber laser cavity 1500 comprising a pump laser diode1505 which delivers pump light at a pump wavelength into a wavelengthdivision multiplexer 1507 designed for the pump wavelength and cavityoperating wavelength. The pump light is then injected into the opticalcavity and into a length of RED fiber (Ytterbium in this example) 1522which provides optical gain. The amplified optical power (emission fromthe Ytterbium-doped fiber) propagates into the single-mode input of asubstantially passive up/down TOF device 1550) as described in previousembodiments of the present invention. The output of the TOF device, inthis case also a single mode optical fiber, is spliced to a chirpedoptical fiber Bragg grating 1513 which reflects light at a pre-definedoptical wavelength determined by the design of the grating.

Reflected light from the grating propagates back through the variousfibers and into a fiber coupler device 1580 with coupling ratio as perthe optimized design of a cavity. By way of example, the chosen couplerratio in FIG. 15 is 20:80 in which 80% of the optical power propagatinginto the coupler device input at port 1581C is directed to port 1581Band 20% to port 1581A. The 80% output port 1581B is connected to asaturable absorber 1515 in this example a SESAM. 20% of the power exitsthe cavity through coupler port 1581A. Pulsed light reflected from theSESAM propagates back through the coupler device into the cavity withsome of the power lost through coupler port 1581D, which can be used formonitoring purposes.

In various practices of the disclosure any optical apparatus describedherein may be configured to provide pulses having a time duration of nogreater than 500 ns; no greater than 100 ns; picoseconds time duration,defined herein as 1 ns or less; no greater than 200 ps; no greater than100 ps. In combination with any of the foregoing, the pulses can be noshorter than, for example, 1 ps or 500 fs. In other practices, the pulsecan be no less than 500 fs and no greater than 100 or 200 ps. It will beappreciated that the examples provided in this application relate tofiber lasers, such as pulsed fiber lasers, yet this is not a limitingcase and the same inventive process applies, as one example, to laserssuch as Q-switched lasers, gain switched lasers, and other types oflaser systems, including those based on a MOPA architecture.

What is claimed is:
 1. A laser system for generating optical pulses atan operating wavelength of the laser system, the system having anoptical resonator comprising: first and second reflectors; and a taperedoptical fiber disposed between the first and second reflectors andhaving a core which has a tapered first section which tapers from asingle mode to multimode at the laser operating wavelength and a secondsection of substantially constant diameter capable of supportingmultiple modes at the laser operating wavelength and an output, whereinthe tapered optical fiber is not tapered down or is tapered down suchthat at the output the core can support a plurality of modes; whereinthe laser system provides output pulses have a repetition rate of nogreater than 10 MHz.
 2. The laser system of claim 1 wherein saidresonator comprises a mode locked resonator.
 3. A laser system forgenerating optical pulses at an operating wavelength of the lasersystem, the system having an optical resonator comprising: first andsecond reflectors; a length of ytterbium doped optical fiber disposedbetween the first and second reflectors for providing optical gainwithin the ytterbium gain bandwidth; a tapered optical fiber disposedbetween the first and second reflectors and having a core which has atapered input section which tapers from single mode to multimode at thelaser operating wavelength, an inner section of substantially constantdiameter capable of supporting multiple modes at the laser operatingwavelength and a tapered output section which tapers from a firstdiameter to a second diameter that is smaller than the first diameter;wherein said tapered optical fiber provides normal dispersion at theoperating wavelength of the laser system; wherein the length of thetapered optical fiber comprises at least 80% of the length of theoptical resonator; and wherein the laser system is configured to provideoptical pulses having pulse repetition rate of no greater than 15 MHzand a time duration no greater than 1 ns.
 4. The laser system of claim 3wherein said resonator comprises a mode locked resonator.
 5. The lasersystem of claim 4 wherein one of said reflectors comprises asemiconductor saturable absorber mirror.